Method to reduce cross scan error visibility during manufacture of a hologon

ABSTRACT

By modifying the exposure sequence of the facets during manufacture of a hologon image writer the visibility of cross scan errors are minimized, thereby reducing exposure error and visible banding. With hologons having a number of facets &#34;n&#34; the general form of the exposure sequence is 1, n, 2, n-1, 3, n-2 . . . .

REFERENCE TO RELATED CASES

Reference is made herein to commonly-assigned, copending U.S. patent application Ser. No. 874,789, entitled "A METHOD OF MAXIMIZING THE FREQUENCY OF ADJACENT FACET CROSS SCAN ERROR SUCH THAT THE INCREASED SPATIAL FREQUENCY REDUCES THE VISUAL PERCEPTION OF THE EXPOSURE ERROR", in the name of B. Narayan and J. E. Roddy, filed concurrently herewith.

FIELD OF THE INVENTION

This invention relates generally to holography, and pertains more particularly to the manufacture of a multi-faceted holographic beam deflector element and method of making such an element so as to reduce cross scan error visibility.

BACKGROUND OF THE INVENTION

A hologon is a device which, when rotated, causes a stationary beam directed at it to repetitively scan along a line. Hologons comprise a disc mounted for rotation at high speed. The disc has, on a planar surface to which the axis of rotation is perpendicular, a plurality of sector-shaped facets. Each facet contains a diffraction grating. If there are "n" facets, the inclination of grating lines in one facet to the lines in adjacent facets (360/n)°. The lines may be tangential or "radial". If "radial", the lines of a facet are parallel to a radius which bisects the facet. If tangential, the lines are perpendicular to the radius which bisects the facet. The gratings may be reflective or transmissive. Multifaceted hologons having "n" facets cause "n" scans per revolution.

It is known to make a hologon by coating a disc with photoresist or other photosensitive material such as dichromated gelatin. A radiation beam, usually a laser beam is split and the resulting two beams are interfered at the photoresist coating so that the coating is exposed to a rectilinear interference pattern. The extent of the latent image created is limited by a mask which has an aperture bounded in part by two radially inner and radially outer boundaries and opposed lateral boundaries radial to the axis of rotation of the disc. The angle included between this line is the same as the included angle of the facet, i.e., (360/n)°. After one facet has been exposed, the disc is rotated through an angle of (360/n)° relative to the mask and means for directing the radiation beams at the disc, and another facet is exposed. After all "n" facets have been exposed, the coating disc is processed to produce operative gratings from the latent images in the photosensitive coating.

In various types of flying spot scanners, cross scan error, unwanted beam deviation perpendicular to the direction of beam scan, causes an undesirable artifact in the written image, termed banding. It is the nonuniform spacing of scan lines that causes the visual appearance of dark and light bands in an image area that would normally be a uniform gray level. Banding related to flying spot scanners comes primarily from two sources: facet-to-facet non-uniformity in reflectivity or diffraction efficiency and facet-to-facet pointing error. This present invention deals exclusively with the latter. In mirror type scanners (polygons), the mirror facets are not parallel to the axis of rotation causing clustering and spreading of the written scan lines. A similar, but much reduced, problem occurs in hologons where the grating pitch varies slightly from facet-to-facet because of subtle changes that have occurred in the hologon fabrication fixture while the facets are being exposed. The consequence of non-uniform grating pitch from facet-to-facet on the hologon is non-uniform line spacing when the hologon is used in a laser writer.

Depending upon available laser power and photoresist sensitivity, each hologon facet takes many seconds to minutes for exposure. During the time for fabricating an entire disc, thermally induced drift in the laser cavity or in the exposure fixture can cause creep in the laser wavelength or fixture geometry. The result is a slow drift in the grating pitch as successive facets are made. This drift can result in a ramp-like variation in cross scan error (FIG. 7) with successive facets around the disc. The error between adjacent facets is small until the large discontinuity between the last facet and the first facet. Such an error can produce noticeable banding because error between adjacent facets has the most effect on local exposure.

A number of methods are known in the art for correcting cross scan error in multifaceted rotating scanners. One method taught by Fleischer in U.S. Pat. No. 3,750,189 was to use combinations of cylindrical and toroidal lens elements to focus the beam on the mirror surface in one dimension and to relay that image to the final plane. This passive technique allows substantially more error in the scanning element, reducing the cost of the element itself, but toroidal lenses are difficult to make and align and the amount of correction afforded is limited in both scan angle and resolution. An active correction system such as acousto-optic or electro-optic deflection for a hologon scanner has been taught in U.S. Pat. No. 4,786,126 to Kramer. Use of such a system adds cost and complexity to the equipment and generally requires measurement and testing of each scanning element. Additional optics are often required to get the beam into and out of the deflection system.

The three patents cited below apply to the fabrication of hologon scanner discs and assume everything is perfect in the laser and exposure fixture and deal with other details of the exposure without altering the sequence of facet exposure as taught by the present invention.

U.S. Pat. No. 4,787,688 to Rumfola teaches a method to improve duty cycle by overlapping facets.

U.S. Pat. No. 4,455,061 describes a "facet" which is made up of a number of small areas of different plane wave (linear) holograms thereby providing a complex scan pattern with the "facet". The hologram is linearly translated, e.g., left to right to accomplish the scan and then must be reset to the original starting position. The reciprocating action required for this is slow, complex and generally suffers from a poor duty cycle when compared to the rotational motion that is more commonly employed. In a laser printer, a fast, linear scan with a large duty cycle is required. Arbitrary scans may be more useful in conjunction with optical memories, laser radars, or target designators. A linear scan could be accomplished using this technique. Because it uses numerically sequential exposure of facets and subfacets, the problem of thermal drift will be the same in the laser and exposure fixture as described previously.

U.S. Pat. No. 4,747,646 rotates a hologon disc mounted on a motor at high speed to gyroscopically stabilize it to eliminate decenter problems. Decenter is a major problem because of a focusing (nonlinear) hologon facet. A hologon made with a linear facet is very insensitive to such error and does not require such an expensive technique for facet exposure. The exposure appears to be sequential and there is nothing in the exposure sequence to compensate for thermal problems.

SUMMARY OF THE INVENTION

The present invention describes a method of reducing cross scan error visibility without special passive or active components. Only a modification of the facet exposure sequence during the manufacture of the hologon is required. Performance in terms of banding can be dramatically improved without requiring an increase in cost of the writing equipment.

The present invention provides a method of manufacturing a hologon to reduce cross scan error, said hologon having an axis of rotation and a number of "n" greater than one, of generally sector-shaped facets uniformly disposed about said axis, the hologon being intended to have incident upon a substrate having a photosensitive layer receiving beams of radiation directed to the photosensitive layer to create an interference pattern on the layer in the sector-shaped facet. The improved method of manufacture comprises rotating the hologon through angular steps of (360/n)° and then exposing the sector-shaped facets to the beams of radiation in the sequence of 1, n, 2, n-1, 3, n-2 . . . where n is the facet number which sequentially increases around the disc.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 represents a plan view of a hologon;

FIG. 2 is a cross-sectional view taken on line 2--2 in FIG. 1;

FIG. 3 is a schematic representation of an apparatus for manufacturing hologons;

FIG. 4 is a view of a mask included in the apparatus illustrated in FIG. 3;

FIG. 5A shows the exposure profiles, line by line, where all facets are "perfect". The exposures for each shown plot with respect to arbitrary units of intensity from 0 to 100;

FIG. 5B shows the sequential line by line exposure resulting in a uniform exposure or gray field;

FIG. 5C illustrates the introduction of a cross scan error of 20% of a pixel or 1/5 of a line spacing such that the line from facet #3 is closer to the line from facet #2;

FIG. 5D illustrates graphically the effect of integrated exposure where the lines are closer together and the exposure rises by as much as 20%;

FIG. 6A illustrates a cross scan error of 40% of a pixel;

FIG. 6B illustrates an integrated exposure within variations of ±40% of normal;

FIG. 6C illustrates a cross scan error of 100% of a pixel;

FIG. 6D illustrates the integrated exposure within variations ±100% of nominal when compared with FIG. 6B, it can be seen that as line placement error increases, the exposure variations increase;

FIG. 7 shows a ramp function where the cross scan error is arbitrarily set to zero for facet 1 and increases monotonically to the last facet;

FIG. 8 illustrates one way to minimize facet-to-facet variations by generating a move smoothly varying function. The dotted line illustrates a portion of a sinewave;

FIG. 9 illustrates a modified exposure sequence to reduce adjacent facet error.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIGS. 1 and 2, there is illustrated a hologon 20 which comprises a glass disc 22 with a coating 24 of photoresist. The disc 22 has an aperture 26 coaxial with the geometric axis 28 of the disc 22, for receiving a drive shaft (not shown) to which the hologon is secured for rotation in unison with the shaft.

The hologon 20 illustrated in FIG. 1 has six facets F₁ -F₆, each subtending an angle of 60° at the axis 28. There are radial boundaries B₁₂, B₂₃, B₃₄, B₄₅, B₅₆ and B₆₁ between the facets F₁ and F₂, F₂ and F₃ and so on, respectively. Each facet has a radially inner boundary 23 and a radially outer boundary 25 both of which are arcs of respective circles. The facets may be termed generally sector-shaped even though they are sectors truncated by the boundaries 23.

The photoresist coating 24 in each facet has been exposed and processed to exhibit a diffraction grating pattern. The diffraction grating is a plurality of straight, closely spaced lines 30 (only a very few of which are shown in the drawing) which, when light is incident on the grating, causes diffraction of the light. In the present example, the lines 30 in each facet F₁ -F₆ are parallel to a line R₁ -R₆, respectively, which is a radius bisecting the respective facet. In other examples, the lines in each facet are termed tangential and are perpendicular to the lines R₁ -R₆ in each respective facet.

The diffraction grating lines 30 in each facet F₁ -F₆ are formed by interfering two coherent beams so that an interference pattern is created on the photoresist. The interference pattern creates in the photoresist a latent image of a grating pattern. A mask limits the extent of the interference pattern. After one exposure, there is relative displacement so that another facet can be exposed. After all facets have been exposed, the photoresist is processed and the grating patterns result.

FIG. 3 illustrates schematically an apparatus 32 for creating the grating pattern latent image in the photoresist coating 24. The apparatus includes a rotary table 34 mounted from a base 38, for rotation about an axis 36. Drive means 40 are provided for driving the table 34 in rotation and for holding it in desired positions.

A prism assembly 42 is mounted from the base 38 by support structure 44. The prism assembly 42 has a beam splitting interface 46 between two prisms 48 and 50. The plane of the interface 46 contains the axis 36.

A mask 52 is disposed between the prism assembly 42 and the disc 22 and is supported by the support structure 44.

A laser 54 provides a source of coherent, substantially monochromatic light which is directed as collimated beam 56 at a face 58 of the prism assembly 42.

The mask 52 is illustrated in FIG. 4 and consists of an opaque plate 60 having an aperture 62. The aperture has a radially inner, smaller diameter circular arcuate boundary 64 and a large diameter, radially outer circular arcuate boundary 66. The circular arcuate forms of the boundaries 64 and 66 have a common center 68. The other two boundaries 70 and 72 of the aperture 62, termed herein opposed lateral boundaries, are rectilinear and, as shown, are radial to the circular arcuate forms of the boundaries 64 and 66. The mask 52, supported by the supportive structure 44, is disposed with the center 68 on the axis 36.

For forming a hologon, a glass disc 22 with photoresist coating 24 thereon is positioned on the table 34. The laser 54 is energized and its output beam 56 enters the prism assembly 42 through face 58. The beam is incident on the interface 46 which splits the beam into beam 74 and beam 76. Beam 74 is reflected off face 78 and leaves the prism assembly 42, as beam 80, through face 82. Beam 76 is reflected off face 84 and leaves the prism assembly 42, as beam 86, through face 88.

The prism assembly 42 is so located and the beam 56 is of such cross-sectional size and shape that the beams 80 and 86 fill the aperture 62. The beams 80 and 86 interfere and form an interference pattern on the photoresist coating 24. The interference pattern is a plurality of bright and dark parallel lines which are parallel to the radius R₁ from the axis 36, which radius bisects the angle included between opposed lateral boundaries 70 and 72 of the aperture 62 in the mask 52. After an appropriate duration of exposure, the exposing beam 56 is interrupted by a shutter (not shown) and the drive means 40 is energized rotating the table 34 on the base 38. In the present example, the hologon to be produced has six facets, therefore, the drive means 40 rotates the table through exactly 60° i.e., (360/6)° and holds the table 34 in its new position. The exposure process is repeated and the table is again stepped.

Gratings in general and hologons in particular can be generated interferometrically (holographically) in photosensitive media to form either a volume grating or a surface relief grating. Both are fabricated by generating two laser beams and intersecting them at a specific angle to provide interference fringes. A photosensitive material on a suitable substrate is placed in the beams at the point of interference. The angle between the beams is varied to change the desired grating pitch.

In a volume grating, a material such as dichromated gelatin is coated as an emulsion on glass. When exposed to the laser and then developed, it exhibits regions of two different indices of refraction corresponding to the bright and dark bands of the interference pattern which provides exposed and non-exposed regions, respectively. These regions are distributed through the volume of the emulsion, and serve to diffract the laser beam when used in the writer equipment.

In a surface relief grating, photoresist is coated onto a glass substrate, exposed, and developed. In a typical photoresist, the emulsion hardens where exposed and remains after development. The resist in the unexposed areas is washed away. The result is a surface relief grating with regions of air and photoresist providing regions of two different indices of refraction.

Both types of gratings can be used to make hologons. Because drift in the laser wavelength on the angle between interfering laser beams is the source of the cross scan error problem, performance of both types of gratings will be improved by the novel exposure method disclosed herein.

As pointed out earlier, the facet-to-facet cross scan error for adjacent facets has the most effect on local exposure variation. However, average cross scan or pyramidal error is often quoted because it yields a lower value, but it should be recognized that one bad facet in an otherwise "perfect" 10-faceted scanner can have a low average error but still have objectionable banding. Peak-to-peak cross scan error is not the best way to characterize the amount of banding either, unless it happens to involve adjacent facets.

The adjacent facet cross scan error is the best descriptor of banding because it is the best indicator of exposure variation. For simplicity, assume that all facets of the scanner are "perfect" in both efficiency and beam pointing except facet 3 which exhibits known amounts of beam pointing (cross scan) error only. Further assume that a uniform gray field is being written and that the exposure profile is triangular--a reasonable approximation to the Gaussian profile normally encountered. FIG. 5A shows the exposure profiles, line-by-line, where all facets are "perfect". The exposure for each line is shown superimposed in arbitrary units from 0 to 100. To obtain the integrated exposure for any given location, the sequential line-by-line exposure must be added yielding the result in FIG. 5B of a uniform exposure or gray field. Of course, for a given photoreceptor, both the gamma and toe and shoulder nonlinearities should be taken into account to arrive at a transmittance or reflectance value.

FIG. 5C introduces a cross scan error of 20% of a pixel (1/5 of a line spacing) such that the line from facet 3 is closer to the line from facet 2. FIG. 5D shows the effect of integrated exposure. Where the lines are closer together, the exposure rises by as much as 20%. Where the lines are spaced further apart than normal, the exposure is reduced by 20%. FIGS. 6A and 6C, respectively, illustrate cross scan errors of 40% and 100% of a pixel. FIGS. 6B and 6D show the integrated exposure within variations ±40% and ±100%, respectively, of nominal.

It should be noted that as line placement error increases, the exposure variations increase. The larger the exposure deviation, the more visible it becomes. FIGS. 5A through 6D illustrate how cross scan error in just one facet can cause visible banding and that the cross scan error between adjacent facets dominates the variation in exposure which produces the visible effect of banding.

Based on the foregoing illustrations, if the adjacent facet cross scan error can be minimized, then the variations in exposure and, therefore, the visibility of banding artifacts can be reduced. In the manufacture of hologons, one can reduce the adjacent facet cross scan error for a given process without reducing the average cross scan error for the disc. The use of the present invention means that the performance of the hologon in terms of visible banding can be improved without making major improvements in the hologon manufacturing process.

This method of reducing cross scan error visibility assumes that the hologon manufacturing process has a repeatable error function, such as shown in FIG. 7. The function need not be a ramp but simply one which repeatedly has a large adjacent facet error which can be reduced by reshuffling the exposure sequence. Of course, if the cross scan error as a function of facet exposure sequence is totally random from disc to disc, then the process is not well enough in control for this method to help. The underlying assumption is a well-controlled process with a repeatable error trend.

As can be seen in FIG. 7, the cross scan error can show a repeatable trend from disc to disc. This Figure illustrates a ramp function where the cross scan error is arbitrarily set to zero for facet 1 and increases monotonically to the last facet, in this instance, facet 8. Thermal drift in the hologon fabrication equipment can cause such an error pattern. When such a hologon is used to write an image, the facet-to-facet cross scan discontinuities are small until facet 8 followed by facet 1, which has a large discontinuity in cross scan error and, therefore, a large variation in exposure. It has been determined that if the writing order of the facets could be rearranged such that facets 1 and 8 are not together, then the exposure variation could be reduced. One way to minimize facet-to-facet variation is to generate a more smoothly varying function, such as a portion of a sinewave, as illustrated by the dotted line in FIG. 8. A way to approximate a more smoothly varying function is to vary the exposure sequence of the disc, as illustrated in FIG. 9. Facet 1 is exposed first, the last facet is exposed next, then facet 2, then the next to the last, facet, then facet 3 and so on. Going around the disc from facet 1 through 8 as shown in FIG. 8, the position in the exposure sequence counts upward through the odd values (1, 3, 5, 7) and then in reverse order through the even values (8, 6, 4, 2). The resultant cross scan error is shown by the solid line in FIG. 8. It should be noted that the peak-to-peak error and the average error haven't changed but that the adjacent facet error has been significantly reduced, from 7 units to 2 units, a factor of 3.5.

This exposure sequencing method assumes that the variation in cross scan error as shown in FIG. 7 indicates a drift with time in the exposure equipment, probably a thermal drift. This can result as the equipment gradually warms up, the angle between the two interfering laser beams can drift which gradually changes the grating pitch and, consequently, the diffraction angle changes. Another potential contributor is the laser cavity itself. The cavity length gradually expands with increasing temperature, resulting in a slight drift in the wavelength of the laser during hologon exposure. A change in laser wavelength causes a change in grating pitch and, consequently, the diffraction angle of the writing beam. One or both of these effects could contribute to the ramp function observed.

Reducing the adjacent facet cross scan error reduces the exposure variation and reduces the visibility of the banding. Resequencing the exposure reduces the error between adjacent facets and the amount of reduction is dependent upon the number of facets (n) because the exposure time and, therefore, the drift in facet pitch depends directly on the number of facets.

Assuming a ramplike function for cross scan error, selection of an optimum exposure sequence is straightforward. The cross scan error with the new sequence should start low, build to a maximum, and end low, approximating a smooth, sinusoid-like function. For an 8-faceted scanner, for example, facet 1 is exposed first and is arbitrarily assigned zero error. To avoid a large cross scan error between facet 8 and facet 1, facet 8 will be exposed next. Since it is the second facet to be exposed, it will have a cross scan error of 1 unit (FIG. 7). Facet 2 will also be adjacent to facet 1 and should be exposed third to keep its cross scan error low (2 units for the third facet exposed--FIG. 7). The sequence continues: first facet, last facet, second facet, next-to-last facet, until all facets are exposed. The last facet exposed will be facet 5 which will have a cross scan error of 7 units. The adjacent facets 4 and 6 will have high cross scan errors also, 6 and 5 units, respectively, so that the difference in error between adjacent facets is low.

It should be understood that the cross scan error function need not be a ramp to make use of this invention. It need only be a repeatable function which has a large cross scan error between adjacent facets which can be reduced by reordering the exposure sequence to approximate a smooth function of cross scan error versus facet number.

The method for a more generalized error function is:

1) Determine the shape of the cross scan error as a function of position in the exposure sequence (facet number) by producing and testing several hologons using an uncorrected exposure sequence (1,2,3,4, . . . ,n).

2) Select the lowest error facet (or most negative error facet) as a reference.

3) Assign zero error to this facet (position in exposure sequence) making the entire error function positive.

4) To make corrected hologons, expose facet 1 of the new hologon at the exposure sequence position corresponding to zero error. For an increasing ramp function, the first exposure has zero error and facet 1 would be exposed first.

5) Next assign the last facet, facet n, to the exposure sequence position which gives the next-to-lowest error. For a ramp function, facet n is exposed second.

6) Assign facet 2 to the exposure sequence position which gives the lowest remaining error. For a ramp function, facet 2 is exposed third.

7) Select the remaining facets by alternating between the highest numbered facet remaining and the lowest numbered facet remaining assigning the exposure sequence location for the lowest error remaining.

In general, the error reduction factor is on the order of (n-1)/2 because there are n-1 exposure increments and the minimum error is two facet exposure times worth of drift since the exposure delay between any two adjacent facets is typically two facet exposure times. The formula appears to hold for both odd and even number of facets (e.g., a 7-facet or 8-facet hologon).

Table I below presents exposure sequences for hologons where adjacent facet error is minimized. Hologons having both an even number and odd number of facets are shown. This table assumes a ramp-like error function as shown in FIG. 7. Other types of functions can be handled with this technique, but not necessarily with the sequences in Table I. As can be readily seen, reversing the sequences in Table I produce the same result. The object of resequencing the exposure is to approximate a smoothly varying function, such as a sinusoid, in order to minimize adjacent facet error.

                                      TABLE I                                      __________________________________________________________________________     Cross Scan Error                                                                       Number             Adjacent Facet                                              of                 Error Reduction                                                                         Optimized                                                                            Uncorrected                                  Facets                                                                              Exposure Sequence                                                                            Factor   Sequence                                                                             Sequence                             __________________________________________________________________________     HOLOGONS WITH EVEN NUMBER OF FACETS                                                     6   1,6,2,5,3,4   5/2      2 units                                                                              5 units                                       8   1,8,2,7,3,6,4,5                                                                              7/2      2     7                                            10   1,10,2,9,3,8,4,7,5,6                                                                         9/2      2     9                                            12   1,12,2,11,3,10,4,9,5,8,6,7                                                                   11/2     2     11                                           14   1,14,2,13,3,12,4,11,5,10,6,9,7,8                                                             13/2     2     13                                   General Form                                                                           n    1,n,2,n-1,3,n-2 . . .                                                                        n-1                                                                            2                                                   HOLOGONS WITH ODD NUMBER OF FACETS                                                     5    1,5,2,4,3     2        2     4                                            7    1,7,2,6,3,5,4 3        2     5                                            9    1,9,2,8,3,7,4,6,5                                                                            4        2     8                                            11   1,11,2,10,3,9,4,8,5,7,6                                                                      5        2     10                                           13   1,13,2,12,3,11,4,10,5,9,6,8,7                                                                6        2     12                                   General Form                                                                           n    1,n,2,n-1,n-2 . . .                                                                          n-1                                                                            2                                                   __________________________________________________________________________

It should be appreciated that modifications of the sequence provided by those skilled in the art may give equivalent or adequate results. For example, simply reversing the sequence given will provide equivalent performance. The sequence provided is a simple one which will yield minimum adjacent facet error for any number of facets. Obviously, normalizing the cross scan error function to be entirely negative, or bipolar, before selecting a sequence will also yield suitable results.

ADVANTAGES AND INDUSTRIAL APPLICABILITY

This method of manufacturing hologons is intended to reduce facet-to-facet cross scan error in a writer using the hologon and thereby minimizing exposure error and visible banding. The method works for hologons having both an even and odd number of facets.

These sequences need not be restricted to hologons. Any multifaceted scanner such as a polygon or pyramid which exhibits a repeatable error pattern in the manufacturing process such as that shown in FIG. 7 can be resequenced during manufacture to provide less visible banding. 

We claim:
 1. A method of manufacturing hologons to reduce cross scan error visibility by varying the exposure sequence of the facets to minimize adjacent facet cross scan error, said disc having "n" number of facets evenly spaced about the periphery of the disc and coated with a photosensitive material comprising:arbitrarily select a facet on said disc and identify said facet as number one; expose facet number one so that it exhibits the least error value as determined from test studies; identify facet opposite number one as facet number (n/2)+1 for a disc having an even number of facets and (n+1)/2 for discs having an odd number of facets; expose said opposite facet so that it exhibits the highest error value determined from test studies; expose the facets adjacent facet number one with the next lowest error value; expose the facets adjacent said opposite facet with the next highest error value; continue the exposure sequence so that the error value of each facet gradually increases in both directions around the disc from facet number one to said opposite facet whereby the magnitude of the error values about the disc approximates a smoothly varying function.
 2. A method of manufacturing a hologon to reduce cross scan error, said hologon having an axis of rotation and a number "n" greater than one, of generally sector-shaped facets uniformly disposed about said axis, the hologon being intended to have incident upon a substrate having a photosensitive layer receiving beams of radiation directed to said photosensitive layer to create an interference pattern on said layer in said sector-shaped facet, said method comprising:rotating the hologon through angular steps of (360/n)°; and exposing said sector-shaped facets to said beams of radiation in the sequence of 1, n, 2, n-1, 3, n-2 . . . .
 3. A method of manufacturing a hologon having a plurality, "n", of facets to reduce cross scan error comprising the steps of:providing a substrate having an axis about which the hologon is rotated in use; providing a photosensitive layer on said substrate; directing beams of radiation at said photosensitive layer to create a first interference pattern on said layer to form in said layer a precursor of a diffraction grating; masking said beams with a mask whereby said interference pattern is on only a preselected region of said layer having an arcuate segment of approximately (360/n)°, measured at said axis; relatively rotationally displacing said substrate and said beams and mask through angular steps of (360/n)° for exposure of the segments in the following sequence: 1, n, 2, n-2, 3, n-2, . . . , until all of the facets have been exposed; and processing said photosensitive layer whereby diffraction gratings are created in the photosensitive layer by the interference patterns.
 4. The method as claimed in claim 3 where n equals 10, the exposure sequence is 1, 10, 2, 9, 3, 8, 4, 7, 5,
 6. 5. A method as claimed in claim 3 where n equals 9, the exposure sequence is 1, 9, 2, 8, 3, 7, 4, 6,
 5. 6. A method as claimed in claim 3 where n equals 11, the exposure sequence is 1, 11, 2, 10, 3, 9, 4, 8, 5, 7,
 6. 7. A method of manufacturing a hologon to reduce cross scan error, said hologon having an axis of rotation and a number "n", greater than one, of generally sector-shaped facets uniformly disposed about said axis, the hologon being intended to have incident upon it, at a radial distance "r" from said axis, a beam of radiation which is converted from stationary to scanning by rotation of the hologon about said axis, comprising:providing a substrate; providing a photosensitive layer on said substrate; directing a beam of radiation at said photosensitive layer to create an interference pattern on said layer; masking said beams with a mask having an aperture bounded in part by first and second bonding edges which are radial or approximately radial to said axis, the angle subtended at said axis by an arc at said radius "r" and extending between said first and second boundary edges occupying a generally sector-shaped region having an accurate extent, at said radius "r", subtending an angle of (360/n)° at said axis; relatively rotationally displacing said substrate and said beams and mask through angular steps of (360/n)° for exposure of the segments in the following sequence: 1, n, 2, n-1, 3, n-2, . . . until all of the facets have been exposed; and processing said photosensitive layer to form diffraction gratings.
 8. A method as claimed in claim 7 where n equals 8 and the exposure sequence is 1, 8, 2, 7, 3, 6, 4,
 5. 9. A method as claimed in claim 7 where n equals 7 and the exposure sequence is 1, 7, 2, 6, 3, 5,
 4. 10. A method as claimed in claim 7 where n equals 6 and the exposure sequence is 1, 6, 2, 5, 3,
 4. 11. A method as claimed in claim 7 where n equals 5 and the exposure sequence is 1, 5, 2, 4,
 3. 